ElementAt and ElementAtOrDefault

ElementAt will return the item at index n. If n is not within the range of the enumerable, throws an ArgumentOutOfRangeException.

int[] numbers  = { 1, 2, 3, 4, 5 };
numbers.ElementAt(2);  // 3
numbers.ElementAt(10); // throws ArgumentOutOfRangeException

ElementAtOrDefault will return the item at index n. If n is not within the range of the enumerable, returns a default(T).

int[] numbers  = { 1, 2, 3, 4, 5 };
numbers.ElementAtOrDefault(2);  // 3
numbers.ElementAtOrDefault(10); // 0 = default(int)

Both ElementAt and ElementAtOrDefault are optimized for when the source is an IList<T> and normal indexing will be used in those cases.

Note that for ElementAt, if the provided index is greater than the size of the IList<T>, the list should (but is technically not guaranteed to) throw an ArgumentOutOfRangeException.